Method and a system for hot hydrostatic pressing

ABSTRACT

A method for hot hydrostatic pressing by means of working liquid which tends to become unstable within a transition time when in contact with a blank heated to a high working temperature and at a pressure lower than the liquid&#39;s stabilization pressure. The pressing of the blank heated to the high working temperature is carried out by raising the pressure of the working fluid to said stabilization pressure for a time shorter than the transition time. This is achieved by surrounding the heated blank by a cover adapted to delay the contact between the heated blank and the working liquid until the pressure in the working chamber is raised.

FIELD OF THE INVENTION

[0001] This invention relates to methods of extrusion and isostaticpressing and particularly to methods using high-temperature pressing.

BACKGROUND OF THE INVENTION

[0002] The invention belongs to the methods of hot hydrostatic pressing,wherein the blanks are heated to temperatures above the 600-700° C.range, such as hot extrusion of metals and sintered billets, HIP-ping(Hot Isostatic Pressure), hot compacting of powders and deposited powdermaterials, plastic deformation under high isostatic pressure, etc.

[0003] In the practice, hot blanks heated up to the above temperaturerange are pressed using such working liquids as special silicon oils andheat-resisting grease. At a higher temperature range, in the so-called“forging interval” about 1000-2000° C., it is known to press blanks andmaterials in a media of gases or colloid graphite.

[0004] Pressing in inert or reactive gases is performed in pressurizedautoclaves (gasostats) at pressures up to 1500-2000 atm and temperaturesup to 2000° C. A basic shortcoming of the “gas” pressing is therelatively low attainable pressure and the high complexity of theequipment. The power consumption per unit weight of the blank is highdue to the great compressibility of the working gas and theimpossibility to recuperate the compressed gas energy. Since the blanksare heated inside the gasostat, the working gas and the autoclavechamber are heated as well, contributing to energy losses.

[0005] The low attainable pressure in the “gas” pressing is a majorlimitation for such pressing methods as HIP-ping and compactingprocesses, where it significantly prolongs the seasoning time of theblank. A typical gasostat working cycle in such a process takes 6-8hours. Besides, the low pressure prevents obtaining of high-qualitycompacted powders and deposited materials.

[0006] The pressing of hot blanks in “pseudofluid” media such as colloidgraphite may be carried out at very high pressures and isenergy-efficient since the “pseudofluid” has low compressibility.However, this media is characterized by a considerable internalfriction, which makes the pressure non-uniform over the working volume.Colloid graphite is also prone to undesirable chemical reactions withthe blank material.

[0007] JP 01269509 discloses a method of using a low-temperature workingliquid for pressing resin powder at high temperature by filling theheated powder in a heat-insulated capsule with heat-insulated rubbercover and pressing the capsule in a hydrostatic press by means of saidworking liquid. Here the capsule and the cover protect the workingliquid from contact with the heated powder during the whole processingtime. Consequently, the method is limited to temperatures that a rubbercover may endure, e.g. 500-600° C.

SUMMARY OF THE INVENTION

[0008] The following is a Glossary of terms used in the presentdescription and claims:

[0009] Hot hydrostatic pressing—a pressing operation on a heated blankby means of a working liquid, such as hot extrusion, sintering,compacting, hot isostatic pressure, etc.

[0010] Blank—any material, blank, detail, or preparation thereof in acontainer, capable of being pressed in heated state.

[0011] Contact—an immediate contact of the blank and the working liquidor a contact via a media not inhibiting substantially the heat transferbetween the blank and the working fluid.

[0012] Working temperature—a temperature rendering the blank susceptibleto hot hydrostatic pressing, i.e. making the blank plastic, sinterable,compactable, etc. at a given working pressure.

[0013] Working pressure—the pressure needed for hot hydrostatic pressingoperation at a given working temperature.

[0014] Unstable (state of working liquid)—a state of a working liquid,wherein it evaporates vigorously, boils, inflames, explodes, chars, etc.on the surface of a heated blank and is thereby incapable of uniformlytransmitting external (working) pressure to the blank.

[0015] Stable (state of working liquid)—a state of a working liquid,wherein it is capable of uniformly transmitting external (working)pressure to the blank.

[0016] Stabilization pressure—the minimal pressure for a giventemperature, at which a working liquid is in stable state in thevicinity of a surface of a blank heated to that temperature.

[0017] High working temperature—working temperature in hot hydrostaticpressing at which the working liquid tends to transit into unstablestate if its pressure is lower than the stabilization pressure.

[0018] Transition time—the time period during which a working liquidbrought in contact with a blank heated to high working temperature, atpressure lower than the stabilization pressure, transits from stable tounstable state in the vicinity of the contact surface.

[0019] In accordance with the present invention, there is provided amethod for hot hydrostatic pressing using a working liquid which tendsto become unstable within a transition time when in contact with a blankheated to a high working temperature and at a pressure lower than theliquid's stabilization pressure, wherein the pressing of the heatedblank is carried out at a working pressure not lower than thestabilization pressure, and the pressure of the working fluid is raisedto said stabilization pressure during a time period after the contactthat is shorter than the transition time.

[0020] This method allows the usage of cheap working liquids such asmineral oils with low internal friction for high-temperature andhigh-pressure hydrostatic pressing. The working liquid is of lowcompressibility and does not pose limitations to raising the workingpressure. The method is energy efficient since only the blank needs tobe heated; it is time efficient since the processing time, e.g. forcuring defects, is less at higher temperatures and pressures, and thereis no waiting period for cooling down of the press equipment betweenprocessing cycles.

[0021] It is known that when a liquid like oil or water is brought incontact with a hot surface, the liquid starts to evaporate veryintensively forming a gaseous “cushion” at the contact interface. Thegaseous cushion has low thermal conductivity, which retards furtherevaporation. If the surface is very hot, the liquid may start to boil,to burn, to explode, or to char, thus destroying the interface gaseouscushion in a short transition time. However, if the pressure of theliquid is sufficiently high, the gaseous interface layer may bestabilized even on a very hot surface. Based on this phenomenon, themethod of the present invention is to raise rapidly the pressure of theworking fluid, after it comes into contact with the hot blank, and reacha high stabilization pressure in a time shorter than said transitiontime.

[0022] The value of the stabilization pressure depends on the kind ofworking liquid and the blank temperature. Practically, it appears that apressure of the order 6000-8000 atm is sufficient.

[0023] The transition time after the contact between the working liquidand the hot blank, in which the stability of the gaseous layer is lostif the stabilization pressure is not attained, also depends on the blanktemperature and the working liquid properties. Practically, “safe” timesfor attaining the stabilization pressure appear to be less than tenthsof a second.

[0024] In order to reduce the time for raising the pressure, thevelocity of the press plunger has to be quite high, at least during thepressure gain stroke. This velocity depends on the volumes of thechamber and the blank, on the specific pressing process, etc. and inpractice appears to be about 150-500 mm/s. During this time the powerplant of the press must deliver huge power rates, for example, for a1200 t press facility, at 200 mm/s velocity, the required power ratewill be more than 2500 kW. A short-time pulse of such power rate can beconveniently delivered by a hydraulic press with a powerful accumulationstation. A screw press with a flywheel may be also used if the flywheelis able to accumulate the necessary energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to understand the invention and to see how it may becarried out in practice, a preferred embodiment will now be described,by way of non-limiting example only, with reference to the accompanyingdrawings, in which:

[0026]FIG. 1 is a schematic view of system for hot hydrostatic extrusionaccording to the invention;

[0027]FIG. 2 is a first scheme for HIP-ping according to the invention;and

[0028]FIG. 3 is a second scheme for HIP-ping according to the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0029] The method of the invention will be explained herein by means ofthree embodiments, with reference to the above drawings.

[0030]FIG. 1 shows a system for hot hydrostatic extrusion according tothe invention and stages of the extrusion process performed on a heatedblank 2 by means of a working liquid 4 and a metallic cover (capsule) 6.The extrusion is carried out in a hydraulic or other press having aworking chamber 10, a plunger 12 movable into the chamber and sealedthereto by a seal assembly 14. The working chamber 10 is closed by anextrusion die 16 with an extrusion opening 18 and a guiding surface 20.The system is equipped with devices, not shown in the drawing, forheating the blank 2, for inserting the blank 2 and the capsule 6 in theworking chamber 10, and for feeding the working fluid 4 into the workingchamber.

[0031] At stage (a), a heated blank is inserted in the working chamber10. At stage (b), the capsule 6 is inserted to encase the blank with aloose fit but tightly fitting the extrusion die 18. At stage (c), theworking chamber is filled with the working liquid 4, and at stage (d),the plunger 12 moves into the working chamber 10 rapidly raising thepressure at least to the stabilization pressure.

[0032] When, at stage (d), the working liquid pressure rises, themetallic capsule 6 shrinks tight around the blank and starts to transmituniformly the external pressure there upon. Also, a heat contact betweenthe heated blank and the working liquid is established and intensiveheat transfer starts via the metallic capsule 6. The heat transfer doesnot develop before the shrinkage, due to the insulating air gap betweenthe capsule and the blank. From the moment of contact on, the pressuremust be raised to the stabilization pressure during a time less than thetransition time to the state of liquid instability.

[0033] If the extrusion process requires a working pressure higher thanthe stabilization pressure, then the plunger moves further. Under thehigh working pressure, the blank 2 is extruded through the die opening18, conducted by the guiding surface 20. Depending on othertechnological needs, the capsule 6 may be extruded together with theblank to form a surface layer thereon (as shown in FIG. 1d), or mayremain and be smashed in the working chamber. In fact, the role of thecapsule is to delay the moment of contact between the hot blank and theworking liquid until a pressure level close to the stabilizationpressure is reached, such that further raise of pressure to thestabilization pressure can take less time than the transition time.After the moment of contact, the capsule may be destroyed.

[0034]FIG. 2 shows a scheme for HIP-ping performed on a heated blank 2by means of a working liquid 4 according to a second embodiment of theinvention. The HIP-ping process is carried out in a hydraulic or otherpress having a working chamber 10 with a forechamber 30, a plunger 12movable into the forechamber and the working chamber through a sealassembly 14. The working chamber 10 is separated from the forchamber 30by a breakable membrane 32, and is closed by a flange 34. The system isequipped with devices, not shown in the drawing, for heating the blank2, for inserting the blank 2 into the working chamber 10, and forfeeding the working fluid 4 into the forechamber 30.

[0035] In this case, the forechamber 30 is filled with working fluid 4beforehand, the blank 2 is heated and inserted into the working chamber10 with flange 34 removed. Then the flange 34 is secured in place andthe plunger 12 is moved into the forechamber 30 to raise the pressuretherein. Upon reaching some predetermined breaking pressure, themembrane 32 breaks open and lets the working fluid flood the workingchamber 10 instantaneously and come into contact with the hot blank 2.The plunger 10 continues its motion and raises the pressure at least tothe stabilization pressure level. Since, after membrane has been broken,the working liquid pressure is due first to fall, the breaking pressureis preferably higher than the stabilization pressure, such that afterflooding the working chamber, the pressure can reach the stabilizationpressure during a time less than the transition time to the state ofliquid instability. Advantageously, in this embodiment, the plungerstarts the final compression stroke from a position very close to theblank and with some accumulated inertia, which reduces the necessarytime for achieving the stabilization pressure.

[0036] After establishing the required working pressure, the plungerstops and the HIP-ping process is carried out for the required timeduration.

[0037]FIG. 3 shows a scheme for HIP-ping according to a third embodimentof the invention. The HIP-ping process is carried out in a hydraulic orother press having a working chamber 10 with a plunger 12 movable intothe working chamber through a seal assembly 14. The plunger carries atits leading end a container 36 made of thin deformable material andfilled with working liquid 4. The working chamber is closed by a flange34.

[0038] A blank 2 is heated and inserted into the working chamber 10 withflange 34 removed. Then the flange 34 is secured in place and theplunger 12 with the container 36 is moved into the working chamber 10.The container butts into the blank and starts to deform. Upon reachingsome predetermined breaking pressure, the container 36 breaks open andlets the working fluid 4 flood the working chamber 10 instantaneouslyand come into contact with the hot blank 2. The plunger 10 continues itsmotion and raises the pressure at least to the stabilization pressurelevel. The further operation of this embodiment is essentially the sameas in the second embodiment.

[0039] It is understood by a person skilled in the art that theabove-described features of the invention may be easily combined withinthe frame of the basic ideas of the invention. For example, the breakingmembrane or the breaking container of the second and third embodimentmay be used in the hot extrusion process of the first embodiment, or theheated blank may be inserted in the working chamber either from above orfrom beneath.

1. A method for hot hydrostatic pressing by means of working liquidwhich tends to become unstable within a transition time when in contactwith a blank heated to a high working temperature and at a pressurelower than the liquid's stabilization pressure, wherein the pressing ofthe blank heated to said high working temperature is carried out by theworking liquid at a working pressure not lower than said stabilizationpressure, and the pressure of the working fluid is raised to saidstabilization pressure during a time period after said contact that isshorter than said transition time.
 2. A method for hot hydrostaticpressing according to claim 1 carried out over a heated blank placed ina working chamber of a press filled with the working liquid, andcomprising the step of providing conditions for reducing said timeperiod.
 3. A method for hot hydrostatic pressing according to claim 2,wherein said step includes surrounding the heated blank by a coveradapted to delay the contact between the heated blank and the workingliquid until the pressure in the working chamber is raised to such levelthat, after said contact has been established, a further raise ofpressure to the stabilization pressure takes less time than thetransition time, said cover also being adapted to allow a uniformtransmission of pressure from the working fluid to the blank after saidcontact has been established.
 4. A method for hot hydrostatic pressingaccording to claim 2, wherein said step includes: forming a forechamberadjacent to the working chamber and isolated therefrom; filling saidforechamber with said working liquid; raising the pressure of theworking liquid in the forechamber to such level that, after said contacthas been established, a further raise of pressure to the stabilizationpressure takes less time than the transition time; opening a passage forthe working liquid from the forechamber to the working chamber.
 5. Amethod for hot hydrostatic pressing according to claim 2, wherein theraise of pressure is performed by an arrangement having moveablemechanical parts and wherein said step includes setting said mechanicalparts in motion, after which the inertia of said mechanical parts isused to accelerate said raise of pressure.
 6. A system for hothydrostatic pressing of a blank at a high working temperature by themethod of claim
 1. 7. A system for hot hydrostatic pressing according toclaim 6, comprising a press with a working chamber, said chamber havinga working liquid inlet for receiving the working liquid, and beingadapted to receive at least one blank, a high-pressure arrangementcapable of raising the pressure of the working liquid in the workingchamber at least to said stabilization pressure during a time shorterthan said transition time.
 8. A system for hot hydrostatic pressingaccording to claim 7, wherein said high-pressure arrangement comprises apressure accumulation forechamber sealingly separated from said inlet bya membrane, said membrane is adapted to break when the pressure in theforechamber reaches a predetermined level, thereby flushing the workingliquid into the working chamber.
 9. A system for hot hydrostaticpressing according to claim 6, comprising a press with a working chamberadapted to receive at least one blank, a plunger capable of moving alongthe working chamber with sealing fit thereto, a sealed container withworking liquid secured to the plunger at the side of the received blank,said container being adapted to break after colliding with the blankwhen the pressure in the container reaches a predetermined level,thereby flushing the working liquid into the working chamber, and saidplunger being capable of further moving after flushing the workingchamber, thereby raising the pressure of the working liquid at least tosaid stabilization pressure during a time shorter than said transitiontime.
 10. A system for hot hydrostatic pressing according to claim 6,comprising a high-pressure arrangement with moveable mechanical partsadapted to be set in motion and accelerated before the working fluid isbrought into contact with the heated blank, and to use thereafter theinertia of said motion for raising the pressure of the working liquid atleast to said stabilization pressure, during a time shorter than thetransition time.
 11. A system for hot hydrostatic pressing according toclaim 6, further comprising a deformable cover adapted to surround theheated blank and to allow a uniform transmission of pressure from theworking fluid to the blank at least after the stabilization pressure isestablished, and a device to install said cover around the heated blank.12. A system for hot hydrostatic pressing according to claim 11, whereinsaid system is adapted to perform hot extrusion, the working chamber hasa die opening with a fitting element around said opening at the internalside of the chamber; either the deformable cover or the blank is adaptedto sealingly fit the fitting element, thereby enabling the extrusion ofthe blank through the die opening.